1. Field of the Invention
The present invention relates to a Group III nitride semiconductor single crystal, to a method for producing the same, to a self-standing substrate, and to a semiconductor device. More particularly, the present invention relates to a Group III nitride semiconductor single crystal produced through a flux process, to a method for producing the same, to a self-standing substrate, and to a semiconductor device.
2. Background Art
Semiconductor crystals are produced through vapor-phase growth methods such as metal-organic chemical vapor deposition (MOCVD) and hydride vapor phase epitaxy (HVPE); molecular beam epitaxy (MBE); and liquid-phase epitaxy. One liquid-phase epitaxy technique is a flux process employing Na flux.
In a general procedure of the flux process, a gallium nitride (GaN) layer is formed on a sapphire substrate or a like substrate, to thereby prepare a seed-crystal substrate, and a semiconductor single crystal is grown on the seed-crystal substrate. In this case, the seed-crystal substrate, raw materials of the semiconductor single crystal, and a flux are placed in a crucible, and then the semiconductor single crystal is grown while the temperature and pressure inside the reaction chamber are controlled. There is disclosed a technique for transferring nitrogen gas from the vapor-liquid interface to the inside of the melt through stirring the melt (see, for example, paragraph [0003], Table 1, etc. of Patent Document 1).
Patent Document 1: Japanese Patent Application Laid-Open (kokai) No. 2010-168236
Meanwhile, for producing a single-crystal substrate having a large diameter, a crucible having a large inside diameter enough for the diameter of the substrate must be employed. In the case where a crucible is rotated at a given rotation speed, the centrifugal force applied to the molten liquid present in the vicinity of the inner wall of the crucible varies in accordance with the inside diameter of the crucible. That is, during rotation of the crucible, the surface level of the molten liquid present in the vicinity of the inner wall rises, as the inside diameter of the crucible increases.
In one case, a semiconductor single crystal having a flat surface can be produced through rotating, at a predetermined rotation speed, a crucible having a small inside diameter. However, when a semiconductor single crystal is grown through rotation, at the same rotation speed, of a crucible having a larger inside diameter, the obtained semiconductor single crystal has an indented center part. Thus, even when the rotation speed is tuned so as to suitably grow a semiconductor single crystal, the surface of the semiconductor single crystal may be flat or non-flat, depending on the diameter of the employed crucible. Therefore, difficulty is encountered in growth of a semiconductor single crystal having a large diameter and having a flat surface.